Development
Sea lice have both free swimming (planktonic) and parasitic life stages. All stages are separated by moults. The development rate for L. salmonis from egg to adult varies from 17 to 72 days depending on temperature. The life cycle of L. salmonis is shown in the figure; the sketches of the stages are from Schram.
Eggs hatch into nauplius I which moult to a second naupliar stage; both naupliar stages are non-feeding, depending on yolk reserves for energy, and adapted for swimming. The copepodid stage is the infectious stage and it searches for an appropriate host, likely by chemo- and mechanosensory clues. Currents, salinity, light, and other factors also will assist copepodids in finding a host. Preferred settlement on the fish occurs in areas with the least hydrodynamic disturbance, particularly the fins and other protected areas. Copepodids once attached to a suitable host feed for a period of time prior to moulting to the chalimus I stage. Sea lice continue their development through 3 additional chalimus stages each separated by a moult. A characteristic feature of all 4 chalimus stages is that they are physically attached to the host by a structure referred to as the frontal filament. There are differences in the timing, method of production and the physical structure of the frontal filament between different species of sea lice. With exception of a short period during the moult, the pre-adult and adult stages are mobile on the fish and, in some cases, can move between host fish. Adult females, being larger, occupy relatively flat body surfaces on the posterior ventral and dorsal midlines and may actually out-compete pre-adults and males at these sites.
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